As a method for the manufacture of flexographic printing plate, there has been used a plate-making process (negative system) where a film of original image is placed on a photosensitive resin layer, the resin of the exposed area is cross-linked by exposing to light and the non-cross-linked resin in the non-exposed area is washed and removed by a developer. Recently however, a method where the information treated on a computer is directly output onto a printing plate to give an uneven pattern which is to be a relief (CTP system) has been diffusing. Particularly, a laser engraving system where engraving is conducted by laser does not need the film of original image and the developing step whereby it is very efficient and environment-friendly method (cf. Patent Document 1) and has been investigated for its practical application.
In a step for the manufacture of printing plate by laser engraving, laser beam is irradiated on a printing original plate based on the image data so that the irradiated area is decomposed and removed whereby unevenness is resulted on the surface of the plate. At that time, residues are formed by the decomposition of the image forming material at the laser irradiated area and a part of them are scattered on the whole printing plate. If the residues are made to leave on the printing plate, some problems are resulted and, therefore, they are removed from the printing plate by sucking them during the laser irradiation with a dust collector installed near the laser apparatus and/or by washing the printing plate after the laser irradiation.
However, since synthetic rubber is a main component in the above printing original plate, tackiness of the plate itself is high, the residues generated by laser irradiation are not able to be removed even by means of suction during the laser irradiation and by washing thereafter whereby there is a problem that they are apt to remain by adhering to the plate. When the residues remain by adhering to the non-irradiated area (convex area) of the printing plate, that results in printing deficiency since ink is applied to such an area during the printing. Moreover, when the residues remain by adhering to the bottom of the irradiated area (concave area) of the printing plate, depth of a screen dot lowers while, when they remain by adhering to the side of the concave area, reproducibility of the screen dot lowers. Any of them may cause the lowering of the resolution.
As a method for overcoming the above disadvantages, there has been proposed an art where a colorless and transparent filler such as fine powder of silica is compounded with the resin composition so as to enhance the mechanical characteristics of the printing original plate and, as a result, the tackiness is lowered (cf. Patent Document 2). However, in a method where a filler such as fine powder of silica is compounded, a lot of the filler is needed for a sufficient lowering of the tackiness of the printing original plate and there is a problem that molding property of the printing original plate and physical property of the plate are significantly deteriorated. As such, addition of a filler causes bad affection on molding property of the printing original plate and physical property of the plate and, therefore, there has been a demand for developing a method by which tackiness of the printing original plate is lowered without addition of the filler.
As a method therefor, the present inventors have already proposed a method where latex is used as a main component for the photosensitive resin composition whereby a flexographic printing plate which results in no printing deficiency and has good resolution is able to be manufactured (cf. Patent Document 3). In the above patent, there is proposed a printing plate giving less residues where a hydrophobic polymer produced from at least two kinds of water-dispersed latexes is made to contain in a printing original plate whereby the ability of forming the image of 150 lpi is achieved.
Recently however, far more highly fine ones have been demanded for printed things and, even in the flexography, there is a need of the printing performance of as far high as 175 lpi as compared with the conventional 150 lpi. To be more specific, the 1% screen dot diameter in 150 lpi is 18.8 μl while that in 175 lpi becomes as fine as 16.1 μm. When such a highly fine printing is to be made possible, an observation under a microscopic has been applied for confirming the image forming property and, in the conventional methods, prevention of printing deficiency due to residues has not been sufficient.